The present invention is generally directed to an RF power amplifier system with an automatic gain control for use in amplifying an RF input signal and is more particularly directed toward amplifier failure compensation.
RF power amplifier systems are known in the art for use in amplifying RF signals for broadcasting purposes, including radio and television. Such power amplifiers may be employed in the broadcasting of either analog television signals, known as the NTSC, PAL, SECAM format, or digital signals, sometimes known as DTV format. When employed in television broadcasting, the frequency bandwidth for the television signals is 6 MHz. The television channels will be in the UHF signal range from approximately 470 to 860 MHz.
The RF input signal to such a power amplifier is obtained from an RF exciter and, for example, this may take the form of a modulated RF carrier within a frequency band of 470 to 860 MHz with the bandwidth of any one channel being on the order of 6 MHz. This RF input signal may have a magnitude on the order of 20 milliwatts. This input signal is increased in magnitude to a much higher level such as on the order of 400 watts which represents a gain on the order of 43 dB.
These power amplifier systems are expected in many situations to be constantly operating so that a radio or television station employing such a power amplifier system may continuously broadcast.
Broad band RF power amplifiers are often required to have a flat gain over the specified frequency range. Usually, an automatic gain control (AGC) closed-loop system is used to control the gain of an amplifier. Most AGC closed-loop controllers are analog, however, the AGC can be digital by using a microprocessor for complex algorithms. A proportional integral derivative (PID) algorithm based AGC closed-loop controller can be used to control the gain of a power amplifier. A PID controller for AGC, which includes both analog and digital components, can automatically adjust the gain, compensating for the variations caused by the temperature changes or a power supply voltage change. It has been widely used in industrial control.
The regular closed-loop AGC control system of a RF power amplifier operating at nominal frequency can be described in the block diagram of FIG. 10 and Equation A.                                                         P              out                        ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                ω                            )                                                          P                              i                ⁢                                  xe2x80x83                                ⁢                n                                      ⁡                          (                              j                ⁢                                  xe2x80x83                                ⁢                ω                            )                                      =                                            G              I                        ·                                          G                C                            ⁡                              (                                  j                  ⁢                                      xe2x80x83                                    ⁢                  ω                                )                                      ·                                          G                P                            ⁡                              (                                  j                  ⁢                                      xe2x80x83                                    ⁢                  ω                                )                                                          1            +                                          G                O                            ·                                                G                  C                                ⁡                                  (                                      j                    ⁢                                          xe2x80x83                                        ⁢                    ω                                    )                                            ·                                                G                  P                                ⁡                                  (                                      j                    ⁢                                          xe2x80x83                                        ⁢                    ω                                    )                                                                                        Equation        ⁢                  xe2x80x83                ⁢        A            
The ideal RF sensor characteristics are:
Vin=GI*Pinxe2x80x83xe2x80x83Expression (1)
Vout=GO*Poutxe2x80x83xe2x80x83Expression (2)
Here,
xcfx89: Frequency (xcfx89=2xcfx80f)
Pin(jxcfx89): Input power signal at nominal frequency
Pout=(jxcfx89): Output power signal from amplifier 602 at nominal frequency
Vin: Voltage signal from input power sensor 600 at nominal frequency
Vout: Voltage signal from output power sensor 620 at nominal frequency
GI=GI(jxcfx89N): Transfer Function of input power sensor at nominal frequency
GO=GO(jxcfx89N): Transfer Function of output power sensor at nominal frequency
GC(jxcfx89): Transfer function of PID controller
GP(jxcfx89): Transfer function for plant (controlled object)
xcfx89N: Nominal frequency
The amplifier itself can contain several sub-amplifiers, which work in parallel. The failure of one of them changes the overall gain. A simple AGC controller would restore the gain level by increasing the RF drive level. That would overdrive the working sub-amplifiers and sacrifice the overall performance. The degradation in performance can not be adjusted nor compensated by the AGC, since the amplifier is driven into a non-linear working region.
A cancellation technique can be used to correct gain variations by correcting for variations caused by sub-amplifier failure.
In accordance with one aspect of the present invention, a power amplifier system is provided for amplifying an RF input signal. The system includes a signal modulator that receives and modifies an input signal and provides therefrom a modified input signal. The system further includes a plurality of M sub-power amplifiers connected together in parallel with each receiving and amplifying a portion of the modified input signal. A combiner combines the amplified portions to provide a combined output signal. A power detector receives the combined output signal and provides therefrom an average output power signal representative of the average output power thereof. A controller monitors the operation of the M sub-power amplifiers to determine if any have failed and adjusts the magnitude of the output power signal to compensate for any failure. The controller controls the modulator in accordance with a function of the magnitude of the output power signal.